Tailoring the degradation behavior and the bioactivity of pure Mg by polycaprolactone/bioactive glass coatings

摘要

Introduction: Polycaprolactone (PCL) is a semicrystalline, bioresorbable material which has been approved by FDA for use in several biodegradable medical and drug delivery devices. PCL has been also considered as a biodegradable coating for controlling Mg dissolution because of its relatively low degradation rate. Bioactive glass (BG) powder can be added into the PCL coating to increase the bioactivity of the coatings. In this study, modified Stoeber method was developed to synthesize a new BG powder to be used in PCL composite coatings. The influence of the BG addition to the PCL coating on the corrosion protection ability of the coating applied for pure magnesium and on inducing formation of hydroxyapatite was investigated by electrochemical and immersion experiments. Materials and methods: Commercially pure magnesium samples in 2-4 mm thickness and 25.4 mm diameter were ground and polished. Finally, they were rinsed with ethanol and dried under hot air. Spin coating was carried out by applying 300μL PCL/BG mixed solution (26.7 wt% BG in coatings) at a speed of 3000 rounds per minute for 30s after 1 min preheating at 120°C. Immersion experiments and electrochemical measurements were performed in DMEM solution at 37 X. Results and discussion: PCL/BG coatings reveal morphology with grain boundaries, which is related to the nature of PCL, and bioactive glass particles or small clusters can be seen accidentally scattered in the coating, microstructure as shown in Fig. 1 (a). After 3 days immersion in DMEM, the flake typical structure of Ca-phosphate (confirmed by FTIR results, not shown here) formed on the surface of magnesium samples as shown in Fig. 1 (b). The formation of such crystalline CaP layer increases the biological reactivity of the samples' surface, which showed both enhance the bonding for bone and prevent further corrosion of the matrix. Fig. 1 Surface morphology of PCL/BG coatings before (a) and after (b) immersion in DMEM for 3 days at 37°C In comparison with bare magnesium, a significant corrosion protection is observed for the PCL/BG composite coated samples, as shown in Fig. 2. The corrosion resistance of PCL/BG coated sample is at least an order magnitude greater than that of the uncoated one, as shown in Fig. 2 (a) and 2(c). Compared with pure magnesium, the potentiodynamic polarization curve shifts down to lower current densities; especially noteworthy is the strong reduction of the anodic current densities. The results of the potentiodynamic measurements show the same trends as the EIS results. Fig. 2 Nyquist (a) plots, Bode plots (b, c) and dynamic polarization curves (d) of PCL/BG coatings in DMEM at 37°C Conclusion: The addition of sol-gel derived bioactive glass into PCL coatings induces flake-like Ca-phosphate formation after 3 days immersion in DMEM. PCL/nBG composite coatings confer magnesium a promising bioactivity and corrosion protection beneficial to biomedical applications.